The present invention relates to a novel aqueous solution for the impregnation of catalyst supports. The invention further relates to catalysts prepared from such a solution and to applications for these catalysts.
The invention relates more particularly to the catalysts used in the hydrotreatment of petroleum fractions, and especially to hydrodesulfurization catalysts.
The feedstocks treated in such hydrotreating processes come, from the fractionation of crude petroleum, for example, by distillation at atmospheric pressure and then under reduced pressure, or from a cracking unit (particularly the fraction known as light cycle oil, or LCO).
The products obtained must be desulfurized before they are used as motor fuel or fuel generally, and current laws impose ever greater reductions of the content of sulfurous and nitrogenous compounds of these products.
To meet these requirements in the area of desulfurization, it is possible to adjust the operating conditions of the hydrotreating process. In particular, the quantity of feedstock being treated may be reduced, which, however, has the effect of decreasing the volume produced by the treating unit and of increasing the operating costs.
Furthermore, the treating capacity may be increased or the operating pressure of the process may be raised. However, this would entail a modification of the installations, and consequently substantial investments and excessive construction delays.
To some extent, the temperature at which the hydrodesulfurization reaction is run could also be increased, but this usually shortens the life of the catalyst employed.
Such catalysts frequently comprise a support based on a porous refractory oxide in which at least one metal from groups VIII, IVa and VIa of the periodic table is present. (See, for example, French patents 2,490,507, 2,560,789, 2,598,631 and 2,598,632.)
These catalysts comprise, for example, an alumina support on which cobalt and/or nickel and molybdenum and/or tungsten, generally in the form of oxides, are deposited at the end of the catalyst preparation treatment.
The life and the activity of such catalysts are limited by a progressive poisoning of the active sites by the sulfur and, depending on the nature of the feedstock treated, also by the nitrogenous products, the metals or the high-molecular-weight products present, which promote the formation of coke.
Apart from the oxides of cobalt (and/or of nickel) and of molybdenum (and/or of tungsten), it is also known to use in such catalysts zirconium oxide, either alone (see French patent 1,437,252) or possibly combined with phosphate (see U.S. Pat. Nos. 3,791,967 and 3,546,105).
The zirconium is incorporated in the catalyst either during the preparation of the support, by coprecipitation with the latter from the same solution, or by being mixed with the support in the form of a solid or paste (see U.S. Pat. No. 4,490,479), or then by impregnation of the support from an aqueous solution (see U.S. Pat. No. 4,344,867).
The last-mentioned method usually involves a double impregnation of the support. (See U.S. Pat. Nos. 1,437,252 and 3,840,473.) The support is first impregnated with a solution of a zirconium salt, this first impregnation being followed by drying and calcination. It is then impregnated in one or more stages with a solution of a nickel or cobalt salt (a nitrate, for example) and with a solution of a molybdenum compound (ammonium paramolybdate, for example), these impregnations being likewise followed by drying and optionally by calcination. While the catalysts so prepared possess satisfactory catalytic properties, their contents of molybdenum and cobalt oxides are generally too low since if present in large amounts these metals, deposited in a second impregnation, might cover the zirconium atoms previously deposited.